We propose to design and build a device that will push the boundaries of existing technology for imaging the activity of large-scale neuronal networks at high speed and single cell resolution. This will be done by using a Multiplexed Scanned Temporal Focusing (MuST) strategy in combination with laser systems with optimized pulse characteristics. Our approach will enable unbiased calcium imaging of unprecedentedly large V-FOVs (500x500x500um at 20Hz, or 1x1x0.7mm at 3Hz) at multi-hertz time resolution. Such an imaging system has the potential to revolutionize neuroscience, providing, for the first time, the ability to monitor the dynamics of network activity of tens of thousands of neurons near-simultaneously. In the mammalian cortex this capability provides the opportunity to gain insights into the computational principles for information processing as it will allow capturing an correlating the dynamics of the network activity across cortical layers. Our technology will be developed through an iterative and in a user-led fashion with our collaborator at Columbia University where it will be made accessible to the neuroscience community. The dissemination of this technique will be further facilitated by providing a dedicated website including open source code and detailed list of materials and instructions for its design and operation.